1. Field of the Invention
The present invention relates in general to valve mechanisms of an internal combustion engine, and more particularly to valve mechanisms of a type which has a plurality of cams for each intake or exhaust valve, the cams being automatically switched in accordance with engine condition.
2. Description of the Prior Art
Hitherto, various valve mechanisms of an internal combustion engine have been proposed and put into practical use, some of which are disclosed for example in Japanese Laid Open Patent Applications 63-167016 and 4-17706, Japanese Laid Open Utility Model Application 63-45521, U.S. Pat. No. 4,768,475, European Laid Open Patent Application 0 276 533, UK Laid Open Patent Application 2 185 784 and UK Laid Open patent Application 2 199 894.
Japanese Laid Open Patent Application 63-167016 discloses a valve mechanism of the above-mentioned type in which two cams are employed for each intake (or exhaust) valve. The two cams have different cam profiles and they are automatically switched to change the valve timing in accordance with engine condition. As is known, in order to improve combustion in an entire operation range of engine condition, is preferable to vary the valve timing in accordance with the engine condition.
One of the two cams (which one will be referred to as the lower speed based cam hereinafter) is shaped to allow the engine to produce a higher torque at relatively low speed, while, the other cam (which will be referred to as the higher speed based cam hereinafter) is shaped to permit the engine to produce a higher torque at relatively high speed.
In addition to these two cams, the '016 publication proposes to use another cam (which will be referred to as the fuel consumption based cam hereinafter), which is shaped to allow the engine to exhibit a marked fuel saving characteristic in a partial load range.
In the valve mechanism disclosed in the '016 publication, a measure is employed in which the cam switching is carried out in such a manner as not to produce a marked difference in output power of the engine between before and after the cam switching. That is, in the disclosed mechanism, the cam switching is effected by selecting a certain engine speed at which the same throttle valve opening degree produces the same outputs of the engine before and after the cam switching.
However, due to its inherent construction, the valve mechanism of the '016 publication has a drawback in which, because the engine torque provided by the fuel consumption based cam is quite small relative to that provided by the low (or high) speed based cam, the cam switching therebetween produces a marked difference in engine torque even at the same throttle valve opening between before and after the cam switching.
As is mentioned hereinabove, in case of switching between the lower speed based cam and the higher speed based cam, the engine speed at which the same throttle valve opening degree produces the same outputs of the engine is used as an index for the switching.
However, in case of switching between the fuel consumption based cam and the low (or high) speed based cam, there is produced a big torque difference upon the cam switching. That is, when using the fuel consumption based cam, the lift of the intake valve is small and the valve closing takes place before the bottom dead center thereby causing the throttle opening to be enlarged for obtaining a needed output torque. Although this fact reduces a pumping loss and thus improves the fuel saving characteristic of the engine, the cam switching from the fuel consumption based cam to the low (or high) speed based cam, with the throttle opening degree kept unchanged, tends to produce a marked switching shock. In fact, upon switching to the low (or high) speed based cam, the engine torque is suddenly increased. In case of switching from the low (or high) speed based cam to the fuel consumption based cam, the engine torque is suddenly reduced. This sudden increase and reduction in torque brings about the marked switching shock.
The cam switching is carried out at will by a driver, that is, by depressing the accelerator pedal. That is, when, under engine operation using the fuel consumption based cam, the accelerator pedal is further depressed for obtaining an output torque beyond the torque actually provided by the fuel consumption based cam, either one of the low and high speed based cams is selected for the cam switching, considering the existing engine speed. Thus, for the reason as mentioned hereinabove, there is inevitably produced a marked shock upon the cam switching.